PAstructural08
Structural
- Apr 21, 2010
- 50
Hello,
I am having a discussion with a few other structural engineers; half agree with me and the other half believe they are correct.
The subject has to do with pressure loads in a duct system (the duct support structure and the duct itself is modeled in STAAD). We know that the duct has a pressure of 20 inches and we calculate the same forces but the way to apply these forces is what we are disagreeing on.
If you see the attachement it is a view a a part of the duct (which is 14 ft in diameter). There is a horizontal component and a vertical component; each component has an cloth expansion joint after the bend with the appropriate supports for the expansion joints. The way the pressure force should be applied (the disagreement) is labeled as #1 or #2.
I beleive it should be #1; as stated in a book by ASCE called "The Structural Design of Air and Gas Ducts for Power Stations and Industrial Boiler Applications;" "Expansion joints are added to a ductwork system to control thermal expansion," which we all know. "Whenever a section of duct has an expansion joint located so that there is a duct wall that does not have another duct wall directly across for it, an unbalanced pressure occurs in that duct section. Unbalanced pressure forces will only occur oppoite openings or expansion joints direction upstream or downstream of bends or transitions." That statement says what I show in the postion of the loads in #1. PS...the flow will be to the right and down the vertical section.
For the defense of the loads at postion #2 here is the response from a collegue;"In the pdf you sent me the pressure distribution shown is correct but its magnitude ((Area duct) x (Patm –Pinternal)) and its direction are easier to determine, without risks of mistakes, when they are assumed to be applied at the EJs and perpendicularly to them. EJs are the only places where the exterior atmospheric pressure is not balanced by the internal pressure (internal pressure is smaller in the case of suction).
I do get confused myself with the orientation of the forces but in case of suction the forces generated (delta between atmospheric pressure and internal ) tend to push the duct toward the EJ.
Also, the system of forces is balanced if we consider it in its entirety, the unbalance exists if we consider partial duct runs, with the exclusion of the forces existing at the interface with fan and bag-house.
My system of pressure forces might not be applied at the points where it’s generated (which is the entire duct surface) but the end resultant is in my opinion correct."
Anyone of any comments??
Thank you
I am having a discussion with a few other structural engineers; half agree with me and the other half believe they are correct.
The subject has to do with pressure loads in a duct system (the duct support structure and the duct itself is modeled in STAAD). We know that the duct has a pressure of 20 inches and we calculate the same forces but the way to apply these forces is what we are disagreeing on.
If you see the attachement it is a view a a part of the duct (which is 14 ft in diameter). There is a horizontal component and a vertical component; each component has an cloth expansion joint after the bend with the appropriate supports for the expansion joints. The way the pressure force should be applied (the disagreement) is labeled as #1 or #2.
I beleive it should be #1; as stated in a book by ASCE called "The Structural Design of Air and Gas Ducts for Power Stations and Industrial Boiler Applications;" "Expansion joints are added to a ductwork system to control thermal expansion," which we all know. "Whenever a section of duct has an expansion joint located so that there is a duct wall that does not have another duct wall directly across for it, an unbalanced pressure occurs in that duct section. Unbalanced pressure forces will only occur oppoite openings or expansion joints direction upstream or downstream of bends or transitions." That statement says what I show in the postion of the loads in #1. PS...the flow will be to the right and down the vertical section.
For the defense of the loads at postion #2 here is the response from a collegue;"In the pdf you sent me the pressure distribution shown is correct but its magnitude ((Area duct) x (Patm –Pinternal)) and its direction are easier to determine, without risks of mistakes, when they are assumed to be applied at the EJs and perpendicularly to them. EJs are the only places where the exterior atmospheric pressure is not balanced by the internal pressure (internal pressure is smaller in the case of suction).
I do get confused myself with the orientation of the forces but in case of suction the forces generated (delta between atmospheric pressure and internal ) tend to push the duct toward the EJ.
Also, the system of forces is balanced if we consider it in its entirety, the unbalance exists if we consider partial duct runs, with the exclusion of the forces existing at the interface with fan and bag-house.
My system of pressure forces might not be applied at the points where it’s generated (which is the entire duct surface) but the end resultant is in my opinion correct."
Anyone of any comments??
Thank you
